1. Field of the Invention
This invention relates generally to oilfield operations and more particularly to an in-line fiber optic amplifier for use with sensors monitoring the condition of downhole equipment, monitoring certain geological conditions, reservoir monitoring and remedial operations.
2. Description of the Related Art
A variety of techniques have been utilized for monitoring wellbores during completion and production of wellbores, reservoir conditions, estimating quantities of hydrocarbons (oil and gas), operating downhole devices in the wellbores, and determining the physical condition of the wellbore and downhole devices.
Reservoir monitoring typically involves determining certain downhole parameters in producing wellbores at various locations in one or more producing wellbores in a field, typically over extended time periods. Wireline tools are most commonly utilized to obtain such measurements, which involves transporting the wireline tools to the wellsite, conveying the tools into the wellbores, shutting down the production and making measurements over extended periods of time and processing the resultant data at the surface. Seismic methods wherein a plurality of sensors are placed on the earth's surface and a source placed at the surface or downhole are utilized to provide maps of subsurface structure. Such information is used to update prior seismic maps to monitor the reservoir or field conditions. Updating existing 3-D seismic maps over time is referred to in industry as “4-D Seismic”. The above described methods are very expensive. The wireline methods are utilized at relatively large time intervals, thereby not providing continuous information about the wellbore condition or that of the surrounding formations.
Placement of permanent sensors in the wellbore, such as temperature sensors, pressure sensors, accelerometers and hydrophones has been proposed to obtain continuous wellbore and formation information. A separate sensor is utilized for each type of parameter to be determined. To obtain such measurements from the entire useful segments of each wellbore, which may have multi-lateral wellbores, requires using a large number of sensors, which requires a large amount of power, data acquisition equipment and relatively large space in the wellbore: this may be impractical or prohibitively expensive.
Once the information has been obtained, it is desirable to manipulate downhole devices such as completion and production strings. Prior art methods for performing such functions commonly rely on the use of electrically operated devices with signals for their operation communicated through electrical cables. Because of the harsh operating conditions downhole, electrical cables are subject to degradation. In addition, due to long electrical path lengths for downhole devices, cable resistance becomes significant unless large cables are used. This is difficult to do within the limited space available in production strings. In addition, due to the high resistance, power requirements also become large.
In production wells, chemicals are often injected downhole to treat the producing fluids. However, it can be difficult to monitor and control such chemical injection in real time. Similarly, chemicals are typically used at the surface to treat the produced hydrocarbons (i.e., to break down emulsions) and to inhibit corrosion. However, it can be difficult to monitor and control such treatment in real time.
Systems for using fiber optic sensors are known in the art. For example, see U.S. Pat. No. 6,281,489 to Tubel et al, assigned to the assignee of this application and incorporated herein by reference. Tubel et al provide apparatus and methods which utilize sensors (such as fiber optic sensors), to monitor downhole parameters and to perform a variety of functions. The sensors are used to measure parameters related to various downhole parameters of interest.
Present day sub-sea production systems can comprise multiple wells connected by flow conduits to a single processing station that can be sub-sea or at the surface. The wells may be separated from the processing station by tens of kilometers. The sensor and communications cables are typically run adjacent, or inside, such flow conduits to a central controller. Optical signals traveling along optical fibers in these cables experience attenuation over the long distances causing poor detection at the central control.
Optical amplifiers are commercially available for the telecommunications industry. These devices, known as erbium doped fiber amplifiers are powered by an electrically energized laser pumping diode to excite erbium ions doped in a small section of the optical fiber to an energized state. An incoming signal causes the excited ions to drop to a lower energy state and emit photons. The pumping diode and associated electronics are connected to optical fiber in the vicinity of the erbium doped section. These commercially available devices are not well suited for the oilfield environment due to their size and the need for electrical power, at the amplifier, to power the laser diode.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing an optical amplifier that requires no electrical energy at the amplifier and is suitable for use in an oilfield environment.